Abstract

We report on the effect of pulse to pulse interactions during percussion drilling of steel using high power ps-laser radiation with repetition rates of up to 10 MHz and high average powers up to 80 W. The ablation rate per pulse is measured as a function of the pulse repetition rate for four fluences ranging from 500 mJ/cm2 up to 1500 mJ/cm2. For every investigated fluence an abrupt increase of the ablation rate per pulse is observed at a distinctive repetition rate. The onset repetition rate for this effect is strongly dependent on the applied pulse fluence. The origin of the increase of the ablation rate is attributed to the emergence of a melt based ablation processes, as Laser Scanning Microscopy (LSM) images show the occurrence of melt ejected material surrounding the drilling holes. A semi empirical model based on classical heat conduction including heat accumulation as well as pulse-particle interactions is applied to enable quantitative conclusions on the origin of the observed data. In agreement with previous studies, the acquired data confirm the relevance of these two effects for the fundamental description of materials processing with ultra-short pulsed laser radiation at high repetition rates and high average power.

Figures (4)

Ablation rate in depth per pulse as a function of the applied repetition rate for different pulse fluences ranging from F = 500 to 1500 mJ/cm2. The ablation depth per pulse is calculated by dividing the total depth of a crater by the number of laser pulses N = 400.

LSM images of ablation craters for different pulse repetition rates. For repetition rates of more than 2 MHz a distinctive ejection of molten material is observed (F = 750mJ/cm2, N = 400). The crater diameter increases from 36.5 µm for the use of 1 MHz up to 49,5 µm for the use of 10 MHz.

Temperature rise as a function of time for z = 1 µm below the workpiece surface for different pulse repetition rates. With increasing repetition rate the temperature rise between two subsequent laser pulses increases due to less time for energy dissipation. The absorbed laser peak-fluence F0 ∙ AHA is 80 mJ/cm2 in this case.

Measured and modeled ablation depth per pulse as a function of repetition rate. The increase is caused by the superposition of heat accumulation and increasing energy input due to pulse-particle interactions.